In Japan, a broadcasting standard called an ISDB-Tn (Integrated Services Digital Broadcasting-Terrestrial narrow) or an ISDB-TSW using an OFDM (Orthogonal Frequency Division Multiplexing) modulation has been proposed as a terrestrial digital audio broadcasting system. Also, in Japan, the “service information for digital broadcasting system, ARIB STD-B10” has been proposed by the association of radio industries and business (ARIB) as the application standard of the configuration of service information, kinds of signals and the data structure and the identifier thereof used for digital broadcasting. There is published a 1.3 edition of the “service information for digital broadcasting system, ARIB STD-B10” as of June, 2000. This “service information for digital broadcasting system, ARIB STD-B10” will be referred to as ARIB STD-B10, hereinafter. According to the ARIB STD-B10, the service information is specified so as to be transmitted by using the data structure of a section type prescribed in the MPEG-2 Systems (ITU-T H.222.0 ISO/IEC13818-1). In the terrestrial digital audio broadcasting in Japan, the configuration of the service information of a broadcasting program is determined in accordance with the ARIB STD-B10. There is published a 3.1 edition of the ARIB STD-B10 which is a revised edition of the 1.3 edition as of Aug. 25, 2000.
When the digital broadcasting of the OFDM system is carried out, prescribed amount of frequency intervals are generally provided between respective channels in order to prevent a radio interference with the adjacent frequency channels to provide guard bands. As compared therewith, the applicant of the present invention proposed a connected transmission method in which adjacent frequency channels are connected in the direction of frequency and an OFDM modulation is performed to the connected frequency channels in the International Patent Application (International Patent Publication No. W000/52861). In the connected transmission method, the center frequencies of OFDM signals in the frequency areas of a plurality of transmission channels are respectively changed and the OFDM signals are multiplexed in the directions of frequencies and the OFDM signals in the frequency areas of the plural transmission channels are simultaneously subject to an IFFT (Inverse Fast Fourier Transform) process. Under the above-described process, the OFDM signals to be transmitted to the plural transmission channels can be multiplexed in the frequency directions, while an orthogonality thereof is maintained.
In the connected transmission method for the OFDM signals, while guard bands between the respective channels are removed, the OFDM signals can be transmitted by connecting the plural transmission channels in the directions of frequency axes and a broadcasting with a frequency availability improved can be performed.
This connected transmission method is currently applied to the ISDB-Tn.
When such a connected transmission is carried out, not only the frequency availability is improved, but also, for instance, a receiving side also has advantageous effects as mentioned below.
For example, when a transmission channel for receiving an OFDM signal is switched to another transmission channel, a receiver cancels a synchronizing channel before switching and tunes in the frequency of a transmission channel after switching. Then, the receiver detects the synchronous code of a transmission control signal (In the ISDB-Tn system, TMCC (Transmission and Multiplexing Configuration Control) corresponds to a transmission control signal) from the transmission channel after switching and carries out the synchronization pull-in operation of an OFDM segment (the frame of transmission data of the OFDM signal). When this synchronization pull-in operation of the OFDM segment is achieved, the receiver can demodulate the signal of the switched transmission channel.
As compared therewith, when the connected transmission is carried out, a transmission signal can be generated while the OFDM segments between channels are synchronized. When the transmission signal is generated while the OFDM segments are synchronized as described above, a receiver can continuously use a synchronizing timing established in a transmission channel before switching also in a transmission channel after switching, even when a channel is switched to another channel between the transmission channels in which the connected transmission is carried out. Therefore, the receiver can simplify the synchronization pull-in operation for demodulation of a signal and perform a channel switch at high speed.
Further, the applicant of the present invention proposed a high speed channel switching upon connected transmission in Japanese Patent Application Nos. 2000-117226 and 2000-117227.
When the modulation system of a transmission channel is a synchronous modulation system, the receiver estimates the transmission characteristics of a transmission line by using SP (Scattered Pilot) signals inserted into the OFDM segments to perform a waveform equalizing process. The SP signal is determined on the basis of the standard of the ISDB-Tn. Since the SP signals are discretely inserted relative to the frequency direction, the receiver interpolates these SP signals in the frequency direction so that the receiver can estimate transmission characteristics to OFDM symbols in all sub-carrier positions in the transmission channel. However, the OFDM symbol located at the end of the frequency direction in the transmission channel has the number of the SP signals referred to for estimation decreased more than that of the OFDM symbol located in the central position of the transmission channel. Therefore, the estimation feature of the transmission characteristics of the OFDM symbol at the end part in the frequency direction in the transmission channel has been deteriorated.
On the other hand, when an upper adjacent channel (a transmission channel adjacent to a high frequency side) of the connected transmission type synchronous modulation system or a lower adjacent transmission channel (a transmission channel adjacent to a low frequency side) of a synchronous modulation system exists in a transmission channel which is receiving a signal, the transmission characteristics can be estimated by using SP signals included in the upper and lower adjacent transmission channels. Accordingly, the OFDM symbol located at the end part in the frequency direction in the transmission channel can also obtain interpolation characteristics similar to those of the OFDM symbol in the central part in the frequency direction of the transmission channel and the transmission line characteristics can be more accurately estimated.
As described above, when the connected transmission is carried out, the receiver side can perform a channel switching at high speed or can improve the estimation characteristics of the transmission line characteristics.
However, it has been hitherto impossible for the receiver side to know information as to whether or not the connected transmission is performed between the transmission channel which is receiving a signal and other transmission channel, and information as to whether the upper adjacent channel is present or the lower adjacent transmission channel is present, and further, information as to whether these channels are of the synchronous modulation system or a differential modulation system.